A typical Worldwide Interoperability for Microwave Access (WiMAX) network based on the IEEE 802.16 standard, includes a set of base stations (BS), and a set of mobile stations (MS) (subscriber stations) served by each base station. A handover from one BS to another can be due to a number of reasons, such as MS mobility, network availability, service availability, network capability, quality of service (QoS), cost, user preference, etc. The handover can be initiated either by the BS or the MS.
There are two types of handover: hard handover and soft handover. In hard handover, the MS first disconnects from the current network and then connects to a target network. In soft handover, the MS first connects to the target network, and then disconnects from the current network.
Service continuity is a key to provide good service to mobile users during the handover. When the MS switches from one BS to another, the handover process should be seamless to mobile users, and ongoing services should not be interrupted. To achieve seamless handover, the amount of time taken for switching network connection must be minimized. According to the ITU TTA evaluation report in May 2007, MS-initiated hard handover takes at least 105 ms without authentication/authorization. WiMAX allows EAP authentication and RSA authorization. EAP authentication process may take seconds to complete as pointed out by R. Fantacci et al, “Analysis of Secure Handover for IEEE 802.1X-Based Wireless Ad Hoc Networks”, IEEE Wireless Communications, October 2007. Therefore, some of network entry procedures need to be done before the handover takes place.
To facilitate the handover between the BSs, Macro Diversity Handover (MDHO) and Fast BS Switching (FBSS) are defined in the WiMAX standard. Both MDHO and FBSS are soft handover protocols. Both MDHO and FBSS are based on a set of BSs, called a diversity set. For the MS, its diversity set changes dynamically. There are several requirements enforced on for both MS and its diversity set in MDHO and FBSS procedures. The MS monitors the BSs and dynamically modifies the diversity set accordingly. BSs in a diversity set share information. There is no authentication or authorization involved in MDHO and FBSS, which can lead to serious security issues. In the invented fast handover protocols, there is no diversity set and handover process can be completed within 100 ms with authentication/authorization for MS-initiated hard handover.
As shown in FIG. 1, a conventional network entry process for a MS 101 and a BS 102 in a WiMAX network. The process includes ten data interchange steps. Such network entry may take hundreds of milliseconds to complete, which is not suitable for transparent handover for a highly mobile station, or an application with a high QoS requirement. These steps are self explanatory, and described in greater detail in the WiMAX standard.